This invention relates to production of a glass optical element such as an optical lens and, in particular, to a press molding apparatus for press molding a preform (obtained by preliminarily forming a glass material) in a heated and softened state to provide a predetermined shape. This invention also relates to a method of producing a glass optical element by the use of the above-mentioned press-forming apparatus.
Recently, in a field of production of an optical element such as an optical lens, it is desired to obtain a high-accuracy lens shape without carrying out surface polishing. To this end, proposal is made of a method comprising the steps of preparing a preform by preliminarily forming a glass material into a provisional shape approximate to a desired shape, heating and softening the preform, and pressing the preform by the use of a pressing mold having a high-accuracy pressing surface (for example, see Japanese Unexamined Patent Publication JP 2001-10829 A).
A press molding apparatus of the type is formed so that a plurality of (for example, four) preforms are simultaneously pressed by the use of a pressing mold comprising an upper mold and a lower mold. The upper and the lower molds are surrounded by an induction heating coil so that the upper and the lower molds are heated to a predetermined temperature by high-frequency induction heating. The upper and the lower molds clamp and press the preforms preliminarily heated and softened to thereby provide each preform with a high-accuracy processed surface.
For example, FIG. 1 shows a basic structure of a typical press molding apparatus of the type mentioned above. The press molding apparatus illustrated in FIG. 1 has a pressing mold comprising an upper mold 502 and a lower mold 504. Each of the upper and the lower molds 502 and 504 has an elongated shape extending in a transversal or horizontal direction in the figure. The upper and the lower molds 502 and 504 are supported by upper and lower supporting members 506 and 508, respectively. The upper supporting member 506 is attached to a fixed shaft 510 while the lower supporting member 508 is attached to a drive shaft 512 of a motor mechanism or the like. The upper and the lower molds 502 and 504 have a plurality of molding portions 514 and 516 formed on confronting surfaces thereof, respectively, to provide preforms with a lens shape. To a position between the upper and the lower molds 502 and 504, the preforms each of which is preliminarily formed into a desired provisional shape are transferred after heated by a heating unit (not shown) to a predetermined temperature, for example, to a temperature corresponding to a viscosity between 105.6 and 109 poises. The upper and the lower molds 502 and 504 are surrounded by induction heating coils 518 and 520 for heating the upper and the lower molds 502 and 504, respectively. The upper and the lower molds 502 and 504, which are preliminarily heated, clamp and press the preforms in a softened state to thereby form high-accuracy processed surfaces on the preforms.
In the meanwhile, upon producing the optical element by precision pressing, accuracy and productivity are important aspects.
In this sense, anisothermal pressing (Japanese Unexamined Patent Publication JP 08-133756 A) has contributed to epoch-making progress. Specifically, by shortening a heating cycle of the pressing mold as compared with existing isothermal pressing, a cycle time required to form the glass optical element can be shortened to the order of several tens of seconds. In addition, surface accuracy and profile accuracy can be kept superior.
Taking the production efficiency into account, attention is directed to a method of obtaining a plurality of optical elements in one heating cycle, i.e., a multiproduct batch process. As far as the heating cycle is essential and requires a predetermined time period, the productivity can be improved if a plurality of optical elements are simultaneously produced in the heating cycle.
In the anisothermal pressing, a glass material is preliminarily heated at a position apart from the pressing mold and thereafter supplied to the pressing mold so that the glass material supplied to the pressing mold is made at a preheat temperature different from that of the pressing mold (frequently, at a temperature higher than that of the pressing mold). In order to avoid product variation, a plurality of glass materials are heated to the same preheat temperature to be uniform in viscosity when they are supplied to the pressing mold. The pressing mold is also heated to a predetermined temperature. In this event, a plurality of molding surfaces must be heated under the conditions as same as possible and, after pressing, must be cooled under the conditions as same as possible. Heating of the molding surfaces under the same condition (i.e., thermal uniformity) is a problem which is not negligible.
As a heat source, use may be made of various options such as resistance heating and induction heating. In order to heat the molding surfaces under the same condition and to avoid occurrence of a local temperature gramoldnt in each single molding surface, the heat source and the molding surfaces are limited in position relative to each other and the molding surfaces are located at an equal distance from the heat source. Thus, upon designing the apparatus, consideration must be made about thermal uniformity. Furthermore, rapid temperature elevation as efficient as possible contributes to the productivity. In view of continuous production, the heating condition must be easily reproducible.
In the meanwhile, one of design options for the pressing mold capable of simultaneously producing a plurality of optical elements is to dispose a plurality of molding surfaces in a single-line arrangement (see JP 2001-10829 A). Such single-line arrangement is advantageous in the following respects. That is, the structure of the pressing mold is simple. In particular, consideration will be made of supply of the glass materials to the pressing mold. In a state where the glass materials are arranged in a single line, a supplying member is split by a straight line into two parts to drop the glass materials through a gap between the two parts. With such a simple mechanism, the glass materials are simultaneously supplied onto the pressing mold (i.e., to the respective molding surfaces).
In order to drop the glass material in a heated and softened state, the glass material in the softened state is floated on a floating saucer by the use of a gas and then dropped and supplied to the pressing mold. This technique is advantageous in that the glass material is stably supplied without damaging the surface of the glass material. For example, by arranging a plurality of floating saucers in a single line and splitting each floating saucer into two parts, the glass materials are simultaneously dropped through gaps between the two parts onto the molding surfaces arranged in a single line. In this case, the apparatus is relatively simple in structure. Thereafter, press molding can be immediately performed before the temperature of the glass material is changed from the preheat temperature. The above-mentioned technique is very advantageous in that the productivity is high and a plurality of optical elements can be stably produced with high accuracy under a thermally uniform condition.
However, the single-line arrangement of the molding surfaces is disadvantageous in the following respect. Specifically, it is difficult to uniformly heat the molding surfaces in the single-line arrangement. Generally, it is difficult to uniformly distribute the heat from the heat source to a plurality of molding surfaces arranged in a single line so that the molding surfaces are heated under the same condition. In case where three or more molding surfaces are arranged in a single line, heat energy supplied from the heat source is different in amount between the molding surface at the center and the molding surfaces at both ends. Such difficulty is encountered by any kind of heat source. For example, even in case where a high-frequency induction heating apparatus extremely high in heat efficiency and high in responsiveness upon temperature elevation and temperature drop (i.e., high in productivity) is used, it is not easy in actual design to arrange the induction heating coil so that all molding surfaces are heated under the same condition.